(a) A Slide Drive Device of a Press Machine Driven by an Electric Servo Motor
Japanese Patent No. 2506657 discloses an electric press for driving a slide directly or indirectly (via a speed reducer etc.) only by using an electric motor (electric servo motor). This electric press can provide high controllability of the slide, but working capacity (energy capacity) which is an important capacity factor for a press or a forming machine can not be secured (insufficient). This means that driving by the electric motor does not have a storage function of energy, and large power can not be continuously discharged due to internal heat generation of the motor, and at forming, an amount of energy provided by the motor is limited.
To solve this problem, it is necessary to prepare an electric motor having a considerably large power (W), and to correspond to it, receiving electricity (facilities) of a user may be huge. Further, during uniform motion of a slide not involving acceleration, deceleration or forming, the electric motor performs only a small workload involved in an extremely small load torque, so that the residual torque (energy) of the electric motor may not be efficiently used.
(b) A Slide Drive Device of a Press Machine Driven by a Variable Delivery Pump+(a Plurality of) Hydraulic Motors (Linked to Each Other in a Closed Circuit)+a Screw
U.S. Pat. No. 4,563,889 discloses a slide drive device of a press machine of which slide is driven by a variable delivery pump+a hydraulic motor+a screw. When this slide drive device of a press machine drives the slide, a problem arises in controllability (responsivity or static accuracies (of velocity or position) of the slide.
That is, because force necessary to drive the slide is proportional to pressure (load pressure) generated from compression of oil flow discharged by the variable delivery pump per unit time in a pipe line connected to the hydraulic motor when load is applied, dynamic characteristics of the slide are lowered by delay in response caused due to this compression (responsivity, or a feedback gain of velocity or position may be lowered). Further, a leakage of pressure oil proportional to the load pressure occurs in the variable delivery pump, the hydraulic motor or valves, so that, especially, accuracies of velocity and position are largely lowered during forming when the load pressure is high. Moreover, because driving is carried out, based on oil flow control by the variable delivery pump, a large amount of oil flowing per unit time becomes necessary, so facilities may be enlarged thereby.
On the contrary, a fly wheel may be provided between the electric motor and the variable delivery pump, and it has a storage function of energy, so that, there is no limitation with respect to energy. Further, there is also a device of the type in that a crankshaft of a press machine is driven by a similar hydraulic circuit (Japanese Patent Application Laid-Open No. 01-309797), but besides the problem described above, problems in control further occur that a transfer characteristic from a drive shaft driven by the hydraulic motor to the slide is nonlinear, and a value of force applied to the slide is limited.
(c) Japanese Patent Application Laid-Open No. 10-166199 discloses a hydraulic drive plastic working device in which an electric motor rotationally drives a constant delivery pump, and a hydraulic cylinder or a hydraulic motor connected to the pump drives a movable platen. This device has a problem that, because a pressure oil medium intervenes in a drive part (because of an effect of compressibility of hydraulic oil or a leakage of pressure oil), controllability included in the electric motor is considerably lowered. Moreover, the problem specific to control of the electric motor that the motor does not have the storage function of energy, and the problem of heat generated in a coil, just as there are, remain. Accordingly, force applied to a press and a work load necessary for press-forming is limited by maximum instantaneous power of the electric motor. An advantage is limited to the point that a system may be simply configured.
(d) Japanese Patent Application Laid-Open No. 2002-172499 discloses a slide drive device which drives a slide via a screw/nut mechanism, by an electric motor and a constant delivery hydraulic pump/motor in parallel. This device is configured in a manner that turning forces applied by both the electric motor and the constant delivery hydraulic pump/motor are combined together and transferred to the screw/nut mechanism.
(e) Japanese Patent Application Laid-Open No. 07-266086 discloses a ram drive device in a plate working machine, in which a direct drive force of a screw pressure device driven by a servo motor, and a direct drive force of a hydraulic cylinder (hydraulic device) including a variable delivery pump or a constant delivery pump as a power source can be transferred to a slide, respectively. In this ram drive device, the screw pressure device mainly positions the ram during a to-and-from drive, and the hydraulic device mainly pressurizes during plate working, and thereby, a high accuracy of positioning can be achieved and a plate can be worked with a large pressurizing force (see the paragraph [0056] in Japanese Patent Application Laid-Open No. 07-266086).
The slide drive device of a press machine disclosed in Japanese Patent Application Laid-Open No. 2002-172499 has the following problem.
(1) Energy Efficiency
A hydraulic motor driven by a constant pressure source has poor energy efficiency, because a leakage of hydraulic oil is large in the hydraulic motor and a friction loss is also large.
(2) Controllability
Drop in controllability (drop in responsivity or limitation to securing a proportional gain in a feedback control configuration) occurs, because an increase in rigidity of the screw/nut mechanism and the drive shaft is caused and moment of inertia converted at an electric motor axis is increased, since turning forces of both the electric motor and the constant delivery hydraulic pump/motor are combined together and transferred to the screw/nut mechanism.
(3) Cost
The constant delivery hydraulic pump/motor is expensive from the viewpoint of marketability or the number of parts.
(4) Noises
The constant delivery hydraulic pump/motor generates pulsing noises at switching between high pressure and low pressure proportional to an angular velocity, and is a noise source.
On the one hand, the ram drive device in a plate working machine disclosed in Japanese Patent Application Laid-Open No. 07-266086 uses the hydraulic cylinder, and so, it does not have the problems (1) to (4) described above. In this drive device, a hydraulic device controls pressure during plate working as described above, and the hydraulic device directly supplies hydraulic oil from the variable delivery pump or the constant delivery pump to an upper room of the hydraulic cylinder. Therefore, it is possible to secure pressurizing force and energy as desired, but problems arise that controllability is considerably lost, because of compressibility of hydraulic oil or a leakage of pressure oil, and further, it is difficult to control the pressurizing force accurately in high responsivity.
Moreover, the hydraulic device described in Japanese Patent Application Laid-Open No. 07-266086 has to drive the variable delivery pump or the constant delivery pump to supply hydraulic oil to the hydraulic cylinder during plate working, and so, also as the motor for driving the pump, a motor having a large power is required.